1. Field of the Invention
The present invention relates to a laser Doppler velocimeter, and more particularly to a laser Doppler velocimeter in which laser light is emitted to at least one point in a region to be measured, and both the flow velocity and the direction of the flow velocity in the region to be measured are measured from a Doppler shift frequency of a signal of the received light. This laser Doppler velocimeter can also be used for a laser Doppler microscope and a laser Doppler radar.
2. Description of the Related Art
A conventional laser Doppler velocimeter (Japanese Patent Application Laid-Open No. 57-59173) which served as a basis of the present invention is shown in FIG. 1. In FIG. 1, reference numeral 1 denotes a laser light source; 14, an optical fiber; 2, a beam splitter; 10, a measuring probe in which a micro lens and an optical fiber are integrated; 11, an optical fiber probe with or without a micro lens; 12, a probe-fixing jig. In addition, reference numeral 13 denotes a micro lens; 6, a light-receiving element such as an avalanche photodiode (APD), a photomultiplier or a pin photodiode (PINPD); 7, a light receiver; 8, a fluid to be measured; and 9, a light-scattering particle.
This laser Doppler velocimeter is based on a differential Doppler heterodyne method in which laser light emitted from the laser light source 1 is split into two projected beams of light by the beam splitter 2, the two projected beams of light are transmitted through the measuring probe and are focused onto a point by means of the micro lenses of the measuring probe, and the flow velocity is detected from a heterodyne detection frequency obtained by performing heterodyne detection by the light receiver 7. In this laser Doppler velocimeter, although the flow velocity can be detected since a Doppler shift appears in proportion to the flow velocity, it is impossible to detect the direction of the flow velocity.
Namely, the frequencies of the two laser beams in a focused portion are identical, and a Doppler shift frequency f.sub.D can be expressed by Formula (2) below. In such a differential-type configuration, however, the direction of the flow velocity cannot be detected since the Doppler shift frequency f.sub.D, which is due to a light-scattering particle moving in the focused portion from top to down in the plane of FIG. 1 and a light-scattering particle moving from down to top, assume identical values.
On the other hand, if the frequency of one of the two split laser beams is set at a value different from that of the frequency of the other laser beam, it is possible to detect a difference frequency of this frequency due to heterodyne detection, so that the signal of the received light from the light receiver 7 is proportional to the flow velocity, as expressed by Formula (1), with this difference frequency set as an offset frequency. Namely, if the offset frequency is known, it is possible to detect the direction of the flow velocity and a flow velocity value.
Accordingly, to detect the direction of the flow velocity, an arrangement is conventionally provided such that a light beam for projection is passed through an acousto-optic element so as to shift the frequency (Japanese Patent Application Laid-Open No. 57-59173, lines 15-17 in the left-hand column of page 3).
In this technique, the frequency f.sub.SA of the signal of the received light from the scattering particle is given by the following formula. EQU f.sub.SA =f.sub.AD +f.sub.D ( 1) EQU f.sub.D =(2v/.lambda.)sin(.phi./2) (2)
where f.sub.AD is an amount of frequency shift by the acousto-optic element, f.sub.D is a Doppler shift frequency due to the scattering particle, v is a flow velocity (vector quantity) of the scattering particle, .lambda. is the wavelength of the light source, and .phi. is an intersecting angle between the two light beams for projection. In other words, since the frequency of the signal of the received light is detected by using the amount of frequency shift f.sub.AD as the offset frequency, if the offset frequency is determined in advance, it is possible to detect only the Doppler shift frequency f.sub.D. Thus, it is possible to determine the flow velocity and the direction of the flow velocity from the detected Doppler shift frequency f.sub.D.
However, since the acousto-optic element is driven stably only at a fixed frequency, the quantity of the frequency shift, f.sub.AD, which is the offset frequency, cannot be set arbitrarily. Hence, the present situation is such that, with respect to a low-velocity object to be measured, high-accuracy measurement is not possible due to a restriction of a Doppler shift frequency analyzer (e.g., a restriction that, when, for example, f.sub.AD =80 MHz, f.sub.D =100 Hz).
To overcome this point, a system has been proposed in "Two-dimensional vector LDV making use of frequency-shift characteristic and self-mixing effect of LD," Extended Abstracts (The 37th Spring Meeting, 1990); The Japan Society of Applied Physics and Related Societies, 28a-c-7. This system makes use of the characteristics of a laser diode in which the oscillation frequency of a semiconductor laser shifts with changes in the temperature and an injected electric current, and if part of the laser light emitted by the semiconductor laser returns to its own oscillation area, there occur such behaviors as the longitudinal-mode hopping, the oscillation of a multiplicity of longitudinal modes, the generation of noise, and the like.
In this system, a two-dimensional vector LDV (laser Doppler velocimeter), such as the one shown in FIG. 2, is configured by confining a frequency modulated LDV, which imparts an offset frequency to a beat signal by making use of the characteristic of a frequency shift with respect to the injected current, and a self-mixing-type LDV making use of interference between return light and oscillation. Incidentally, M1, M2, and M3 denote mirrors, and HM denotes a half mirror. The measurement of a two-dimensional velocity vector becomes possible since the directions of velocity components which are detected by the frequency modulated LDV and the self-mixing-type LDV differ from each other. In addition, the directions of the velocity components can be discriminated by the orientation of the shift of the signal frequency with respect to the offset frequency of the frequency modulated LDV and the orientation of the sawtooth waves observed as signal waves in the case of the self-mixing-type LDV.
Namely, in this system, a semiconductor laser is used as the light source, the oscillation frequency is linearly changed by sawtooth waves by means of an injected current, light is emitted to an object to be measured by means of a differential-type optical system with a difference in the optical path length imparted to the two light beams for projection, part of the light scattered from a passing particle is returned to the semiconductor laser, and a self-mixing effect (Japanese Patent Application Laid-Open No. 2-201165) is made use of, so as to measure the flow velocity and the direction of the flow velocity. At this time, the frequency component f.sub.SM of the detected signal is as follows: EQU f.sub.SM =f.sub.OF +f.sub.D .+-.(f.sub.OF +f.sub.D).multidot.(D/c) (3) EQU f.sub.OF =(d.nu./dt).multidot.(D/c) (4)
where d.nu./dt is a rate of change over time of the oscillation frequency of the semiconductor laser which is the light source, D is a difference between the optical path lengths of illuminating light 1 and 2, and c is the velocity of light in a vacuum.
Since this system is provided with the optical path difference in the atmosphere, the system itself becomes large in size, with the result that the range of the portion to be measured is limited. In addition, since the signal frequency corresponding to the offset frequency fluctuate due to fluctuations in the atmosphere and the vibration of the system, and scattered light from the two projecting optical paths having an optical path difference returns to the light source simultaneously, an output signal due to the self-mixing effect includes frequency components other than those which are due to the Doppler shift and are proportional to the optical path difference between the two optical paths. Consequently, frequency components other than f.sub.OF and f.sub.D appear in the detected signal f.sub.SM, and it is impossible to detect f.sub.D alone on a stable basis, so that it is impossible to conduct signal analysis on a stable basis. Furthermore, since restrictions are imposed on the optical path difference in terms of the structure of the system, there is a problem, among others, in that the range in which accurate measurement is possible is narrow.
In addition, as a velocimeter for simultaneously measuring information on flowrates at a plurality of measurement points, an apparatus is known in which predetermined laser wavelengths are allotted to respective measuring probes, and which employ laser light sources and photodetectors which are each provided in a number identical to that of the measuring probes. However, there is a problem in that the configuration becomes complicated since a plurality of measuring probes and photodetectors are required.
Further, in a laser Doppler velocimeter disclosed in Japanese Patent Application Laid-Open No. 2-107988, by using one laser light source, a laser beam is split into two polarized components perpendicular to each other, the two polarized components are applied to different objects to be measured, the measuring light from the plurality of objects is synthesized with a reference beam, and a plurality of synthesized light components are obtained in correspondence with their polarized states, thereby simultaneously measuring a multiplicity of points by a single optical modulator and a plurality of photodetectors. With this laser Doppler velocimeter, although a single light source is used, a plurality of photodetectors are required for detecting Doppler signals obtained by respective polarized light components, and since the two polarized components which are perpendicular to each other so as not to interfere with each other are separated, only two measurement points can be theoretically measured simultaneously.